Optical fiber network having increased channel capacity

ABSTRACT

Optical communication system apparatus and methods of operating an optical communications system is described. The system advantageously may utilize existing optical fiber networks and provide significantly increased channel capacity. In accordance with one aspect of the invention the system apparatus provides for a plurality of communications channels and a processor unit receives requests for allocation of one or more channels from a node coupled to the optical communications system. The system apparatus dynamically allocates one or more channels selected from unused channels.

FIELD OF THE INVENTION

This invention pertains to optical communications systems, in general,and to an optical network and a method of operating an optical network,in particular.

BACKGROUND OF THE INVENTION

As used herein, the term “optical network” relates to any network thatinterconnects a plurality of nodes and conveys information between nodeswith optical signals. The term “optical communications system” as usedherein refers to any system that utilizes optical signals to conveyinformation between one node and one or more other nodes. An opticalcommunications system may include one or more optical networks.

Telecommunications carriers began installing optical fiber cable about15 years ago. At the time the optical fiber cables were installed, itwas expected that the optical fiber infrastructure would providecommunications systems and networks with ample capacity for theforeseeable future. However, the phenomenal growth of data traffic onthe Internet has taxed the capabilities of the optical fiberinfrastructure. In addition, new high bandwidth applications are beingdeveloped and are being made available for corporate applications. Theresult of this increased usage of the fiber infrastructure is seriousnetwork congestion and exhaustion of the fiber infrastructure. In thepast, optical fiber systems relied on time division multiplexing toroute traffic through a channel. Time division multiplexed systems addmore capacity by time multiplexing signals onto an optical fiber. Adisadvantage of time division multiplex systems is that data must beconverted from light waves to electronic signals and then back to light.The system complexity is thereby increased.

As the demand for increasing traffic capacity continues, the limitationsof existing optical networks and optical communications systems must beovercome. To do so, the capacity of the existing optical networks andoptical communications systems needs to be increased. Capacity ofexisting optical infrastructure may be expanded by the laying of morefiber optic cable, for example. However, the cost of such expansion isprohibitive. Therefore a need exists for a cost-effective way toincrease the capacity of existing optical infrastructure.

Wavelength Digital Multiplexing (WDM) and Dense Wavelength DigitalMultiplexing (DWDM) are being used and/or proposed for use in long-haultelecom network applications for increasing the capacity of existingfiber optic networks. The advantage of both WDM and DWDM is that theconversion to electrical signals is not necessary. The devices thathandle and switch system traffic process light and not electricalsignals. WDM and DWDM would appear to many to be the solution to opticalnetwork limitations. In WDM, plural optical channels are carried over asingle fiber optic, with each channel being assigned to a particularwavelength. By using optical amplifiers, multiple optical channels aredirectly amplified simultaneously thereby facilitating the use of WDMsystems in long-haul optical networks. DWDM is a WDM system in whichchannel spacing is on the order of one nanometer or less. WDM and DWDMexpand the capacity of an optical fiber by multiple wavelength channelsinto a single laser beam. Each wavelength is capable of carrying as muchtraffic as the original. Thus in one example set forth by BarryGreenberg in Special Report: new growth markets/emerging OEMs: lightingthe way to a network-capacity solution, Electronic Buyer News,04-19-1999, pp. 50, “A fiber carrying four 2.5-Gbit/s DWDM channels, forexample, has its capacity increased to 10 Gbit/s, without having toinstall additional fiber of use higher-speed transmission equipment.”With WDM and DWDM, traffic passes from one node of the network to itsdestination in the form of light waves without conversion to electricalsignals. DWDM and WDM will permit increase in the capacity of the fiberinfrastructure. Systems with up to 128 and 240 DWDM channels have beenproposed and/or are being built. However, DWDM and WDM are both limitedby the non-linear cost increase as the network is expanded. In eachinstance, expansion beyond an incremental increase in traffic handlingcapacity may trigger significant investment in new optical fiber andequipment that is significantly in excess of the incremental increase innetwork capacity. In addition, DWDM based systems are not scaleable inexpansion because equipment typically has to be replaced rather thanmerely added to. Existing implementations of both WDM and DWDM are toolimited for solving the congestion problems of the existing opticalinfrastructure. The present systems are limited in the number ofavailable channels. The slight increase in channel occupancy in suchsystems will present severe restrictions on the traffic handlingcapacity of the network. Additional difficulties with presentimplementations of DWDM and WDM technology include lack of flexibility;difficulty in handling packet switched information, non-linear opticaleffects and the already noted lack of incremental and scaleable upgradecapability.

It is therefor highly desirable to provide an optical communicationsystem that has increased channel capacity. It is further desirable toprovide an optical communication system that provides bandwidth basedupon user demand. It is further desirable that any improved opticalcommunication system is able to utilize the existing fiber opticinfrastructure. Such a solution will prevent so called “fiber exhaust”.To effectively utilize the existing infrastructure in the face of thedramatic increases in traffic that will be encountered, it is highlydesirable to increase channel capacity by a factor of 10 to 200 timesthat provided by DWDM to permit up to 20,000 channels to be served. Itis also highly desirable that any improved optical communication systemhas a low cost per channel.

SUMMARY OF THE INVENTION

In a method of operating an optical communications system in accordancewith the invention, a plurality of optical channels is provided and theoptical channels are utilized for communications among a plurality ofcommunications nodes. Each optical channel is determined by at least twoof three optical signal characteristics. A first one of the opticalsignal characteristics is selected from a plurality of predeterminedoptical wavelengths. A second one of the optical signal characteristicsis selected from a plurality of predetermined optical phases, and athird one of said optical signal characteristics is selected from aplurality of optical modulation frequencies.

In one embodiment in accordance with the principles of the invention,two of the optical signal characteristics are utilized to determine theoptical channels. In this first embodiment of the invention, eachchannel is defined by one optical wavelength of a plurality of opticalwavelengths and by one modulation frequency of a plurality of opticalmodulation frequencies.

In a second embodiment in accordance with the principles of theinvention, one wavelength of a plurality of optical wavelengths, onefrequency of a plurality of optical modulation frequencies, and onephase of a plurality of optical signal phases define each channel.

In a system in accordance with the principles of the invention, anoptical network having a plurality of nodes, each node being coupled tothe network, is provided with a laser source that serves as a referenceto synchronize operation of the network. Still further in accordancewith the principles of the invention, the reference laser optical outputis distributed to all nodes of the network. In the illustrativeembodiment of the invention, the reference laser output is distributedvia a separate fiber optic path. By utilizing a distributed referencelaser, a plurality of channels may be defined by.

In accordance with one aspect of the invention, the laser reference isused to generate a plurality of channels for communication paths throughthe network. In accordance with another aspect of the invention, thelaser reference is a multiple wavelength laser.

Optical communication system apparatus and methods of operating anoptical communications system in accordance with the invention mayutilize existing optical fiber networks and provide significantlyincreased channel capacity. In accordance with one aspect of theinvention the system apparatus provides for a plurality ofcommunications channels and a processor unit receives requests forallocation of one or more channels from a node coupled to the opticalcommunications system. The system apparatus dynamically allocates one ormore channels selected from unused channels.

Still further in accordance with the invention, any node coupled to thecommunications system can be coupled to any other node via any unusedchannel. In accordance with one aspect of the invention, the selectionof a channel is in accordance with a predetermined algorithm. Onealgorithm is such that the channel distance between the assigned channeland other channels in use is maximized. Another algorithm is such thatcross channel interference between the assigned channel and channels inuse is minimized.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be better understood from a reading of the followingdetailed description in conjunction with drawing figures, in which likereference designations are used to identify like elements, and in which:

FIG. 1 depicts an optical communication system in accordance with theprinciples of the invention;

FIG. 2 depicts representative optical signal power distribution levelsin a portion of the system of FIG. 1;

FIG. 3 illustrates multiplexing of optical signals in accordance withthe principles of the invention;

FIG. 4 is a block diagram of a system control unit in the system of FIG.1;

FIG. 5 is flow chart illustrating channel assignment in accordance withthe invention;

FIG. 6 is illustrates the wavelength multiplexing utilized in the systemof FIG. 1;

FIG. 7 illustrates use of interferometer technology in an embodiment ofthe invention;

FIG. 8 illustrates a frequency multiplex/switch layer implementationutilized in an embodiment of the invention;

FIG. 9 illustrates in block diagram form a frequency modulator andfrequency demodulator for use in a system in accordance with theinvention;

FIG. 10 illustrates the multiplexing of signals in a second embodimentof the invention in accordance with the principles of the invention;

FIG. 11 illustrates, in simplified form, the transmission of data fromone network node to a second network node;

FIG. 12 illustrates, in simplified.form, the transmission of data fromthe second network node to the first network node of FIG. 11;

FIG. 13 is a block diagram of a network node in accordance with theinvention;

FIG. 14 is a detailed block diagram of a portion of a first opticalnetwork processor (ONP) for use with an embodiment of the invention;

FIG. 15 is a detailed block of a second portion of the first opticalnetwork processor useable in conjunction with the optical networkprocessor portion of FIG. 14;

FIG. 16 is a detailed block diagram of a second optical networkprocessor;

FIG. 17 is a detailed block diagram of a first portion of a thirdoptical network processor for use with the second embodiment of theinvention;

FIG. 18 is a detailed block diagram of a second portion of the thirdoptical network processor useable in conjunction with the opticalnetwork processor portion of FIG. 17;

FIG. 19 is a detailed block diagram of a fourth optical networkprocessor;

FIGS. 20 through 23 depict multiple wavelength laser reference sources;

FIGS. 24 and 25 depict Erbium doped fiber lasers (EDFLs) utilized in thereference sources of FIGS. 20 through 23; and

FIG. 26 depicts an optical add/drop (OAD).

DETAILED DESCRIPTION

Optical networks are increasingly being utilized to supportcommunication of various content between nodes. These optical networksform part of optical communication systems. These optical communicationsystems are, in some applications, characterized as “long-haul”, “metro”and “short haul” networks. In “metro” network applications the bandwidthrequirements of the networks has doubled every ten months. Newcommunications applications require high bandwidth of the type found inMetro networks. However, the present traffic capacity of Metro networksis limited.

In accordance with the principles of the invention a solution to theproblems with existing optical network and optical communication systemscombines bandwidth and channel requirements into a single completearchitecture.

One objective of the present invention is to be able to provide forbandwidth upgrade to the existing optical fiber networks and opticalcommunications systems. In accordance with the present invention,increasing the available channel count and speed increases the bandwidthof existing optical networks. The number of concurrently availablechannels is increased over WDM systems by a factor of 10 to 200permitting serving up to 20,000 channels simultaneously. As will beappreciated by those skilled in the art, the number of users that may beserved by a multi-channel communications system is determined by thenumber of channels and an occupancy factor. In accordance with even amodest occupancy factor and bandwidth demand, the system of the presentinvention may be used to provide communications for in excess of 200,000users concurrently. In addition, the system and method of the presentinvention provides for dynamic, on-demand bandwidth allocation and thecapability of establishing communication between two random nodes, i.e.,any node coupled to the communications system can communicate with anyother node coupled to the communications system.

FIG. 1 depicts an optical communication system 1000 in accordance withthe principles of the invention. Optical communication system 1000includes a Metro Network 1100 coupled to a Long Haul Network 1200 via anoptical add/drop module 1203 and optical cross connect module 1205.Metro Network 1100 couples one or more local access networks 1301, 1303,1305 to each other and to Long Haul Network 1200. Long Haul Network 1200interconnects plural Metro Networks 1100. For purposes of clarity in thedrawings and simplicity in the description, only one Metro Network 1100is shown. Metro Networks 1100 are typically located at widespreadgeographic locations. However, it is not intended to limit applicabilityof the present invention to arrangements in which networks are dispersedgeographically. The present invention is applicable to networks that areoverlapping in geographic areas or even to networks that are in the samegeographic area.

Metro Network 1100 is, in the illustrative embodiment, depicted as aring-based metropolitan network system. Metro Networks are intended toprovide high bandwidth to end customers directly and/or via local loopaccess networks. In the illustrative embodiment depicted in FIG. 1,Metro Network 1100 is depicted as a ring based network having a fiberoptic ring 1101. It will be understood by those skilled in the art thatthe invention is not limited to use in networks that are of a ring basedstructure. The principles of the invention are equally applicable toother network architecture structures including, not by way oflimitation but by way of example, star network structures, mesh networkstructures, and point-to-point structures. For purposes of clarity andbrevity, those additional network architecture structures are not shownin the drawing. In addition, although the illustrative embodiment of theinvention depicts a Metro Network coupled to plural Access Networks1300, the principles of the invention are not so limited. In addition,those skilled in the art will realize that the particular nomenclatureused to describe the illustrative embodiment is also not intended to belimiting of the invention in any manner. For example, it is not intendedto limit any aspect of the invention to so-called “Metro Network”applications. Those skilled in the art are familiar with the specificterminology utilized to describe the illustrative embodiment and willrealize that the invention is applicable to other named communicationsystems and networks. For example, the principles of the presentinvention are applicable to “long haul” networks. Still further, theprinciples of the invention are not limited to optical communicationssystems utilizing only optical fiber for the communications paths. Thoseskilled in the art will recognize that various other terms may be usedto describe or designate the identical or similar networks. For example,the term “long distance network” is also used in place of “long haulnetwork”.

Each Access Network 1301, 1303, 1305 is coupled to optical fiber ring1101 of Metro Network 1100 via “add and drop nodes” referred to hereinas optical add/drops (OADs) 1307, 1309, 1311, respectively. Opticaladd/drops in various forms are known to those skilled in the art. In itssimplest form an optical add/drop is a coupler. Optical add/drops areused to add or extract optical signals. In the present invention, OADs1307, 1309, 1311 are utilized to inject (add) and retrieve (drop)optical signals into and from optical fiber ring 1101. Both add and dropare bidirectional with respect to optical fiber ring 1101. By,“bidirectional” it is meant that optical signals may be transmitted inor received from either direction, i.e., to the right or to the left inthe ring as shown, on optical fiber ring 1101. In addition, OADsutilized in the embodiment of the invention described herein providebroadband operation. An OAD particularly advantageously utilized in theembodiments of the invention is show in FIG. 26 and described in greaterdetail with respect to FIG. 26.

Each optical add/drop 1307, 1309, 1311 is, in turn, coupled to anoptical fiber amplifier 1313, 1315, 1317. Optical fiber amplifiers 1313,1315, 1317 may be of known design. In the illustrative embodiment of theinvention the optical fiber amplifiers 1313, 1315, 1317 are eachErbium-doped amplifiers (EDFAs). EDFAs are the latest state-of-the-artsolution for broad band amplification of optical signals in opticalcommunication systems. EDFAs are commercially available from varioussources. EDFAs overcome propagation losses of the optical signalsthrough the optical fiber and boost the optical signals to necessaryreceiver levels. EDFAs can be used to amplify WDM and DWDM signals.

Access network 1301 includes a plurality of access locations or nodesthat include a residential complex 1331 and a small office building1333. Other access locations are not shown for clarity, but it will beunderstood that more than two access locations may be coupled intoaccess network 1301. Furthermore, it will be understood by those skilledin the art that the various types of access locations or nodes shown aremerely representative of the types of end users and are not intended inany way to limit the scope of the invention. The terms “node” and“access location” are used interchangeably herein. Each access location1331, 1333, 1341, 1343, 1351 has associated with it an optical networkprocessor 1335, 1337, 1345, 1347, 1353. The number of access locations1331, 1333, 1341, 1343, 1351 that may be coupled into an access network1301, 1303, 1305 is dependent upon the number of users and the trafficusage. Access network 1303 includes user complex 1341 and officebuilding 1343 along with other locations that are not shown. Opticalnetwork processors 1345, 1347 are utilized to provide network accessfunctionality for user complex 1341 and office building 1343,respectively. Access network 1305 includes large office complex 1351 anda single optical network processor 1353. It will be understood by thoseskilled in the art that the number of optical network processors 1335,137, 1345, 1347, 135 associated with each access network 1301, 1303,1305 may be more or less than the numbers shown in the drawing Figures.In operation, any user at any of the locations 1331, 1333, 1341, 1343,1351 can utilize the communications system shown to access and exchangeinformation with any other user in access networks 1301, 1303, 1305 orany other user coupled to Metro Network 1100 or to any user coupled tolong haul network 1200. In addition to being able to couple any user toany other user coupleable to the communications system, the system ofthe invention can use any idle channel as a communications channelbetween any two users or nodes. This is identified as the randomconnection capability of the communications system of the invention.

In accordance with the principles of the invention, optical referencesignals originating at a reference laser source are utilized to providefor channel synchronization and to permit a significant increase in thenumber of channels that are available for use in the system. In theillustrative embodiment of the invention, an additional ring is providedfor the distribution of reference optical signals from a reference lasersource. The additional ring serves to distribute reference opticalsignals throughout the Metro Network 1100 to all access networks 1301,1303, 1305. The reference laser source is, in the illustrativeembodiment, co-located with a system control unit 1360. The referenceoptical signals are distributed via a ring network 1370. The referenceoptical signals are coupled to each access network 1301, 1303, 1305 viaan optical coupler 1371, 1373, 1375, respectively. The optical output ofeach coupler 1371, 1373, 1375 is distributed to each optical networkprocessor via an optical amplifier 1381, 1383, 1385. As will be apparentto those skilled in the art, although the structure depicted in theillustrative embodiment of the invention is shown as a ring typedistribution, the invention is equally applicable to other distributionstructures such as, not by way of limitation but by way of example,star, mesh or point-to-point distribution arrangements. Furthermore, thedistribution structure for the reference signals does not have tocorrespond to the structure of the network. That is, because a ringdistribution structure is used in the communications system, a ringdistribution structure does not have to be used with the referenceoptical signal distribution, other distribution structures may be usedincluding hybrid combinations of various distribution arrangements.

The reference laser source utilized in the illustrative embodimentincludes a multiple wavelength laser. To assure adequate optical powerlevels are provided to each node coupled to the access networks 1301,1303, 1305, a distribution network and power allocation arrangement isprovided as shown in FIG. 2. System control unit 1360 has co-locatedtherewith a reference laser source 1362. Optical reference signals fromreference laser source 1362 arc coupled to optical fiber ring 1370.Additional optical couplers 1372 are shown to indicate that additionalaccess networks may also receive optical reference signals. In network1370 additional optical amplifiers 1382 are employed to maintain a powerlevel of +10 dBm for each wavelength. At the output of optical couplers1371, 1373, 1375 the power level is a +0 dBm. Optical amplifiers 1381,1383, 1385 raise the power level to +13 dBm.

The output of each optical amplifier 1381, 1383, 1385 may be distributedat the access network level to one or more optical network processors,such as optical network processor 1335. Optical couplers, such asoptical coupler 1384 provide this distribution. Optical coupler 1384couples the output of amplifier 1381 to up to eight optical networkprocessors, such as optical network processor 1335. The power level foreach wavelength of the reference laser signal at the input to theoptical network processor 1335 is maintained at +3 dBm. By use ofoptical amplifiers 1381, 1383, 1385 and amplifiers 1382 disposed in thereference laser ring network 1370, uniform useable reference lasersignals are made available at each optical network processor. Althoughspecific signal levels are shown in the illustrative embodiment of FIG.2, those signal levels are intended to indicate how distribution ofoptical reference signals at adequate levels may be provided and is notintended to be limiting any way.

In a first embodiment of the invention, a multiple wavelength laser isutilized to provide a reference optical signals for generation andassignment of optical channels that are determined from selecting foreach channel one wavelength of a plurality of optical wavelengths andone frequency of a plurality of optical modulation frequencies. In theillustrative system, the number of wavelengths that are obtainable froma multiple wavelength laser source is M wavelengths, where M is 32. Thenumber of optical modulation frequencies is O, where O is 128. Thus thesystem of the first embodiment of the invention has a channel capacityof 32×128=4096 channels. Each channel in the system of the illustrativeembodiment has a bandwidth of 155 mbs. In other embodiments of theinvention higher or lower speed and bandwidths may be used. Also, inother embodiments of the invention, different numbers of channels,different numbers of wavelengths and different numbers of opticalmodulation frequencies nay be utilized.

Turning to FIG. 3, the functionality of multiplexing and switchingchannels identified by wavelength and frequency is illustrated. For eachwavelength, λ₁ through λ_(M), frequencies F₁ through F_(O), aremultiplexed by multiplexor/demultiplexors 201. The frequency multiplexedwavelengths at the outputs of multiplexors 201 are multiplexed togetherat wavelength multiplexor/demultiplexor 203. The multiplexed opticaloutput of multiplexor/demultiplexor 203 is coupled to optical network1101. The multiplexor/demultiplexor functions are changed todemultiplexing for received optical signals. The optical signalsreceived over optical network 1101 are first wavelength demultiplexed bymultiplexor/demultiplexor 203 to wavelengths λ₁ through λ_(M). For eachwavelength, a multiplexor/demultiplexor 201 demultiplexes thefrequencies F₁ through F_(O). The multiplexor/demultiplexor 201, 203provide switched multiplexing. For example, multiplexor/demultiplexor203 to the left in FIG. 3 can switch any number of wavelengths ontooptical network 1101

FIG. 4 illustrates a system control unit 1360 in block diagram form.System control unit 1360 includes multiple wavelength laser 1362 that iscoupled to optical amplifier 1363. Optical amplifier 1363 coupleswavelength laser 1362 to laser reference ring network 1370. A networkprocessing unit 1364 is provided to control and monitor operation of thesupply of reference optical signals to the reference ring network 1370.A wavelength sensing circuit 1366 is coupled to the output of opticalamplifier 1363. Optical amplifier 1363 provides sensing signals tonetwork processing unit 1364 that permit network processing unit 1364 toadjust the output level of optical amplifier 1363 and to controlmultiple wavelength laser 1362. Network processing unit 1364 is coupledto network 1100 via an optical network processor 1368, an opticalamplifier 1369 and an optical add/drop 1317. Network processing unit1364 receives requests for bandwidth and channel assignments from nodescoupled to the network 1101 and responds with the address of one or moreallocated channels. The number of channels allocated to a node dependsupon the bandwidth needed for handling the traffic. Network processingunit 1364 includes one or more processors and associated memory. Theprocessor units may be commercially available processors. Memoryassociated with the processor unit or units may be any commerciallyavailable memory. Programs stored in memory are utilized to control theoperation of network processing unit 1364.

Operation of system control unit 1360 in processing requests for channelassignments is shown in FIG. 5. System control unit 1360 constantlyidentifies which channels have been allocated and which channels areidle. System control unit 1360 responds dynamically to requests forchannels by selecting channels from the idle channels and allocating thechannels as needed. When communication between users over a channel iscomplete, the channel is returned to the designated idle channel pool.In the illustrative embodiment of the invention, system control unit1360 selects an idle channel to achieve maximum isolation with usedchannels, i.e., the channel is selected to have the maximum separationfrom channels in use. In other embodiments of the invention, the mannerin which channels are selected may utilize a selection algorithm or aweighting selection or other scheme for channel assignment. Inoperation, a system node that needs to transmit information via thenetwork 1100 transmits a request to system control unit 1360 asindicated at step 501, for a channel. The request also identifies thedestination node or nodes. After receiving the request, networkprocessing unit 1364 selects a channel from the pool of availablechannels, as indicated at step 503. The channel address is assigned.Wavelength and frequency identify the channel address. At step 505,network processing unit 1364 provides the designated channel identity tothe transmitting node and to the receiving node. Network processing unit1364 identifies the assigned channel as in use at step 507. Transmissionand reception of information occurs at step 509. Upon completion oftransmission by transmitting node, network processing unit 1364 reclaimsthe channel and again assigns it to the pool of available channels atstep 511. Communication of channel assignments to system nodes may beaccomplished in any one of a number of conventional channel assignmentmethods. In the illustrative embodiment of the invention, communicationof channel assignments to nodes from SCU 1360 and from node to SCU 1360is accomplished by use of dedicated control and communication channels.

FIG. 6 illustrates the operability o f the multiplexing and switchingprovided in improved network of the invention. In FIG. 6, an opticalfiber network such as network 1101 is illustrated as a ring. At thesystem control unit 1360, multiple wavelengths optical signals, λ₁through λ_(M), are multiplexed together and distributed via referencelaser ring network 1370 to network nodes. Chart 602 indicates thewavelengths that are available on reference laser ring network 1370. Anetwork node, identified as node 603 has requested that a channel beassigned. System control unit 1360 allocates a channel. The allocatedchannel includes wavelength λ_(Z). At network node 603, a tuned opticalwavelength filter 605 is utilized to select the wavelength λ_(Z)assigned by the system control unit 1360. Filter 605 couples opticalchannel signals at the wavelength λ_(Z) over the optical fiber network1101. Chart 604 indicates that the output of the output of node 603presented to network 1101 is a single wavelength. Other nodes likewisetransmit different wavelength channels over the network 1101 asindicated by the additional inputs to optical fiber network 1101.Wavelength chart 606 illustrates that although each node nay provide anoptical signal at a single wavelength, optical fiber network 1101carries multiple wavelengths. System control unit 1360 has informed node607 that it is assigned to receive communications from node 603 atwavelength λ_(Z). At a node 607, a tunable optical wavelength filter 609is adjusted to select wavelength λ_(Z) and provide the signal to adetector 611 that is used to extract information carried by the opticalsignals. Chart 608 indicates that the output of tunable opticalwavelength filter 609 provides a single wavelength output. Tunableoutput wavelength filters 605, 609 may be of a design described in theliterature.

In a second embodiment of the invention, advantageous use is made of theproperties of optical signals to further enhance the channel capacity ofoptical communication systems. A phase modulated optical signal may becharacterized in terms its wavelength λ, its phase φ, and its modulationfrequency f. Recognizing this, the second embodiment of the inventionutilizes phase modulated and delayed optical signals and defines eachoptical channel by a wavelength multiplex, a phase delay or coherencemultiplex and a frequency multiplex. In the second illustrativeembodiment of the invention, the number of wavelength multiplexed isidentified as “M”. The number of phase or coherence multiplexed channelsis identified as “N”. The number of frequency multiplexed channels is“O”. With M=32, N=8, and O=64, the number of available channels that maybe multiplexed together is 32×8×64 or 16,000 channels. Each channel hasa bandwidth of 155 mbs. Thus the total bandwidth is 16,000 channels×155mbs=2.5 tbs. In other embodiments of the invention higher or lower speedand bandwidths may be used. Also, in other embodiments of the invention,different numbers of channels, different numbers of wavelengths anddifferent numbers of optical modulation frequencies may be utilized.

The architecture of the second embodiment of the invention is the sameas shown in FIGS. 1 and 2. The system control unit 1360 as shown in FIG.4 and its operation as set forth with respect to FIG. 5 aresubstantially the same in the second embodiment. The system and methodof the second embodiment of the invention utilize a phasemultiplex/switch layer, a frequency multiplex/switch layer and awavelength multiplex layer in addition to the wavelengthmultiplex/switch layer described in conjunction with FIG. 6.

The phase multiplex/switch layer makes advantageous use of the fact thata single wavelength optical signal in optical fiber can carry multiplephases. At the transmission end phase separation is provided throughdelay of the channel with respect to the reference channel. By creatinga phase delay that is larger than the coherence length of the laser,multiple phase channels can be multiplexed into a single wavelength Atthe receive end, phase recovery is provided. By reversing the phasedelay and interfering with the reference signal, the phase-multiplexedchannel can be separated and detected with an interferometer.

Turning to FIG. 7, the manner in which interferometer technology may beutilized to provide phase multiplex/switching is illustrated. In theillustration, four phase channels are illustrated. It will be understoodby those skilled in the art that the number of phase channels shown inFIG. 7 is merely illustrative and is not to in any way be considered aslimiting. In FIG. 7, the transmit end is illustrated at 700 and thereceive end is illustrated at 710. Interconnecting transmit end 700 andreceive end 710 is the optical fiber network 1101. Light source 701generates optical signals. Phase delays are created as shown at 703 toproduce four phase multiplexed data channels. The undelayed opticalsignal 704 provides a reference. The phase delayed signals 702 are phasemodulated at 705 to encode data onto the signals. The phase delayedoptical signals appear on the optical network 1101 as shown at 707. Atreceive end 710, a phase delay reversal is provided at 709. By utilizingreference signals the phase multiplexed reference signal isdemultiplexed. Interferometer techniques 711 are utilized to demodulateand decode the data that was transmitted via the optical signals.

In the phase interferometer multiplex/switching portion, signals aredetected by interferometer based on phase amplitude, not by intensity,to get better sensitivity. Amplifiers can compensate for system lossesthereby leading to a system that tolerates more loss. In addition, byproviding signals in difference phases, multiple wavelength channels arecarried in the same wavelength. Switching between phase channels can bedone electro-optically within less than 0.1 microsecond to allow forfast packet switching. Channel isolation is enhanced by properlyselecting phase, wavelength and modulation frequency.

FIG. 8 illustrates the frequency multiplex/switch layer implementationutilized in the invention. In the illustrative embodiment, O opticalfrequencies, F₁ through F_(O), are utilized as carriers. Modulators 801produce modulated optical signals at the individual optical carrierfrequencies, F₁ through F_(O). Combiner 803 combines the individualcarrier frequencies onto the optical network 1100. As illustrated inspectral chart 804, combiner 803 combines all the carrier frequenciesonto the network optical fiber 1101. At the receive end a divider 805separates the frequency component, F₁ through F_(O). Demodulators 807demodulate the optical signals.

FIG. 9 illustrates in block diagram form a modulator 801 and ademodulator 807. Data 903 to be transmitted is combined in a mixer 905with an TF signal produced by a RF source 901. The resulting RF signalis applied to a RF driver filter 907 that provides appropriate filteringand driver buffering. The output or RF driver filter 907 is supplied tomodulator 909 to modulate an optical signal from a light source 911. Theoptical signal from light source 911 is modulated by an RF signal at themodulation frequency corresponding to the channel assigned forcommunication to the node at which the modulator 801 is located. At thereceiver node that is intended to receive data from the node at whichmodulator 801 is located, demodulator 807 receives optical signals.Demodulator 807 includes a detector circuit 913. Detector circuit 913 isset to detect optical signals at the channel frequency designated forcommunication from the node at which modulator 801 is located. Theoutput of detector 913 is coupled to a RF driver filter 915. The outputof the RF driver 915 is combined with an TF signal provided by RF source917 in a mixer 919 to recover the transmitted data at terminal 921. TheTF signal is at the modulation frequency assigned to the particularchannel.

Turning to FIG. 10, the functionality of multiplexing and switchingchannels identified by wavelength, phase and frequency is illustrated.For each phase, φ₁, through φ_(N), of each wavelength, λ₁ through λ_(M),Frequencies F₁ through F_(O), are multiplexed bymultiplexor/demultiplexors 201. The frequency multiplexed signals foreach of the phases at the outputs of multiplexors 201 are multiplexedtogether at phase multiplexor/demultiplexors 1021. The frequency andphase-multiplexed signals for each wavelength are applied to wavelengthmultiplexor/demultiplexor 203. The multiplexed optical output ofmultiplexor/demultiplexor 203 is coupled to optical network 1101. Themultiplexor/demultiplexor functions are changed to demultiplexing forreceived optical signals. The optical signals received over opticalnetwork 1101 are first wavelength demultiplexed bymultiplexor/demultiplexor 203 to wavelengths λ₁ through λ_(M). For eachwavelength, a corresponding phase multiplexor/demultiplexor 1021demultiplexes phases and for each phase a multiplexor/demultiplexor 201demultiplexes the frequencies F₁ through F_(O). Eachmultiplexor/demultiplexor is bi-directional in that it will switch ormultiplex one or more signals into a single stream and that it willdemultiplex or switch signals out of a combination stream.

Since each channel has a unique wavelength, phase and modulationfrequency correlation, it can be identified by a unique address thatreferences its wavelength, phase and frequency. For M wavelengths, Nphases, and O modulation frequencies each channel may be particularlyidentified by a channel identity in which the wavelength is assigned anumber of from 1 to M, each phase is assigned a number of from 1 to Nand each modulation frequency is assigned a number from 1 to O. Thechannel identity for each channel may be referred to as λ_(m)Φ_(n)f_(o),where “m” is the wavelength number, “n” is the phase number and “o” isthe frequency number. This channel identity is selected for convenienceand clarity in description only and is not in any way intended to limitthe invention.

FIGS. 11 and 12 illustrate the exchange of data between two networknodes as represented by optical network processors ONP#1 and ONP#50.Initially, ONP#1 request a channel allocation from system control unit1360. System control unit 1360 makes the selection of an idle channeland as a result allocates a channel identified as λ₂φ₈F₄ fortransmission of data from ONP#1 to ONP#50. As shown in FIG. 11, ONP#1inserts data. D_(TX) into the designated channel. ONP#50 receives themodulated signal and extracts the data

D_(RX). Upon completion of the data transmission to ONP#50, systemcontrol unit 1360 returns the channel assignment of channel λ₂φ₈F₄ tothe pool of unassigned channels for reassignment. Later, the node atwhich ONP#50 is located requests a channel assignment from systemcontrol unit 1360. System control unit 1360 assigns a channel form thepool of available idle channels. In this instance channel λ₄φ₃F₆ isassigned, ONP#50 transmits and ONP#1 receives data in the assignedchannel. Upon completion of the data transmission, the channel isreassigned by system control unit 1360 to the pool of idle channels.

As shown in FIG. 13, each network node includes an optical networkprocessor ONP that includes a modulator and a demodulator as describedabove. Each ONP is coupled to the laser reference source 1362 via thelaser reference network 1370 as shown in FIG. 1. Each ONP is coupled tothe optical fiber network 1101 via an optical add/drop OAD and anoptical amplifier EDFA.

Optical Network Processor

FIGS. 14 and 15 depict a transmitter portion and a receiver portion ofan optical network processor particularly well adapted for use with theabove-described first embodiment of the invention. FIGS. 16 and 17depict a transmitter portion and a receiver portion of an opticalnetwork processor particularly well adapted for use with theabove-described second embodiment of the invention.

Each optical network processor includes a transmit function and areceive function. The receive function decodes data from a systemscommunications channel assigned for communications to a node coupled tothe optical network processor to control the associated wavelengthmultiplex/switch, phase multiplex/switch and frequency multiplex/switch.The transmit function converts data from an associated node to anassigned system communications channel by controlling the associatedwavelength multiplex/switch; phase multiplex/switch and frequencymultiplex/switch.

Turning to FIG. 14, a transmitter portion of an optical networkprocessor for use in a first embodiment of the invention is shown.Transmitter portion 1400 of an optical network processor includes one ormore processors or micro controllers 1401 that provides program controlof operation of the optical network processor. For clarity only oneprocessor is shown for each optical network processor, but more than oneprocessor may be used. Transmitter portion 1400 is coupled to laserreference network 1370 and receives signals from the multiple wavelengthsignals from laser reference source 1360. A polarization controller 1403under control of micro controller 1401 selects polarization of thereceived laser signals. The output of polarization controller 1403 iscoupled to tunable filter 1407. In an alternate embodiment of theinvention, a depolarizer replaces polarization controller 1403. Microcontroller 1401 receives channel allocation information and utilizes thechannels allocation information to select a wavelength and frequency forits associated node to transmit data. Micro controller 1401 viawavelength tuning module 1405 operates tunable filter 1407. Wavelengthtuning module 1405 selects a wavelength in response to micro controller1401 providing a wavelength select signal. Tunable filter 1407 is tunedto the selected wavelength. Tunable filter 1407 thereby selects thewavelength optical signal for transmitting data under control of microcontroller 1401. The output of tunable filter 1407 is coupled to aMach-Zehnder interferometer 1413. Interferometer 1413 includes two legscoupled at the input to a coupler 1409 and at the output by coupler1419. A first leg includes dc bias module 1412 and a phase modulator1416. A second leg includes dc bias module 1414 and a phase modulator1418. Microcontroller 1401 provides quadrature control of interferometer1413 via bias control module 1411. Quadrature control ensures stablelinear operation of the interferometer 1413. Frequency selection isprovided via microcontroller 1401 controlling voltage-controlledoscillator 1415 that in turn provides a selected modulation frequency tomixer/driver module 1417. Mixer/driver module 1417 mixes the modulationfrequency output of voltage controlled oscillator 1415 with Transmitdata D_(TX). The outputs of interferometer 1413 are provided to tunablefilter 1421 which is tuned by wavelength tuning module 1405 to thewavelength selected by micro controller 1401. The output of tunablefilter 1421 is coupled to network 1101. In addition, coupler 1419 has anoutput coupled to photo detector 1423. The output of photo detector 1423is coupled to micro controller 1401.

FIG. 15 depicts optical network processor receive portion 1500. Receiveportion 1500 of an optical network processor includes a processor ormicro controller 1501 that provides program control of operation of theoptical network processor. Receive portion 1500 is coupled to network1101 and receives signals from another network node. Micro controller1501 receives channel assignment information from SCU 1360 and utilizesthe channel assignment to select the wavelength and frequency of achannel carrying data for its associated node. A polarization controller1503 under control of micro controller 1501 selects polarization of thereceived laser signals. In an alternate embodiment, a depolarizerreplaces polarization controller 1503. The output of polarizationcontroller 1503 is coupled to tunable filter 1507. Micro controller 1501via wavelength tuning module 1505 operates tunable filter 1507.Wavelength tuning module 1505 selects a wavelength in response to microcontroller 1501 providing a wavelength select signal. Tunable filter1507 selects the wavelength of a receive channel under control of microcontroller 1501. A coupler 1509 couples the output of tunable filter1507 to a Mach-Zehnder Interferometer 1513. Interferometer 1513 includestwo legs. A first leg includes dc bias module 1512 and a phase modulator1516. A second leg includes dc bias module 1514 and a phase modulator1518. Interferometer 1513 is not used as an interferometer in thereceiver. Only the dc bias modules 1512 and 1514 are used in the receivefunction. Phase modulators 1516, 1518 are left unused in this receiverimplementation. Micro controller 1501 provides quadrature control viabias control module 1511. Frequency selection is provided via microcontroller 1501 controlling voltage-controlled oscillator 1515 that inturn provides a selected frequency to mixer/driver module 1517. Theoutputs of interferometer 1513 are applied to coupler 1519. The outputof coupler 1519 is in turn applied to tunable filter 1521 which iscontrolled by micro controller 1501 via wavelength tuning module 1505.The wavelength-selected output of tunable filter 1521 is in turn appliedto detector 1523. Detector 1523 provides a quadrature dc output, whichis provided to micro controller 1501 for use in controlling bias controlcircuit 1511. An RF output of detector 1523 is provided to amplifier1525. Output of amplifiers 1525 is coupled to a second input ofmixer/driver 1517. An output of mixer/driver 1517 is applied to low passfilter 1529. The output of low pass filter 1529 provides data outputsignals D_(RX) that are provided to an network node such as user 1331.

As can easily be seen from a comparison of FIGS. 14 and 15, the designof the optical network processor receive portion and transmit portionshare similar basic design components in the implementations shown. Thetransmit portion and receive portions in one embodiment are implementedon tow separate chips for full duplex operation. In another embodimentof the invention, a bidirectional, half-duplex design combines bothtransmit and receive portions in a single integrated optic chip usingreflective design. Advantages of the second embodiment are that thelength of the integrated optic chip is shortened by ½; cost is reduced;and transmit and receive portions are combined into one design. Inaddition, performance of the wavelength filter is greatly enhanced fordouble pass operation. Sidelobe suppression of 15 dB for one passthrough the filter increases to 30 dB with double pass operation. Stillfurther, the drive voltage of the modulator is reduced 50%. A furthersignificant advantage is that integration onto a single chip allowscreation of a large sized phase detector.

FIG. 16 depicts a transceiver 1600 in which a single integrated opticchip 1670 is utilized advantageously. Transceiver 1600 is coupled tonetwork 1101 and laser reference ring 1370. A circulator 1604 and anisolator interposed in the reference laser ring connect transceiver 1600to both. Circulator 1604 is coupled to integrated optic chip 1603 via apolarization controller or scrambler 1603. Integrated optic chip 1670includes a TM polarizer 1651 coupled to a tunable filter 1652. Microcontroller 1601 receives transmit and receive channel assignmentinformation from system control unit 1360 and utilizes the channelassignment information to select wavelength and frequency for transmitor receive functions. Micro controller 1601 via a wavelength-tuningmodule 1605 controls tunable filter 1652. A TE polarizer 1653 followstunable filter 1652 to remove unwanted signals. A 2×2 coupler 1654 isdisposed between TE polarizer 1654 and optical bias modulator 1656.Optical bias modulators 1612, 1614 are followed by phase modulators1616, 1618. Reflection mirrors 1662, 1660 are provided on the end ofintegrated optic chip 1670. The operation of the various circuitelements shown in FIG. 16 is substantially identical to the operation ofthe elements in FIG. 14 for receive operation and to the elements inFIG. 15 for receive operation. There is a one to one correspondence tothe elements of FIGS., 14, 15, and 16 and the operation is identical.

FIG. 17 depicts a transmitter portion 1700 of an optical networkprocessor for use in the above described second embodiment of theinvention. Transmitter portion 1700 includes a processor or microcontroller 1701 that provides program control of operation oftransmitter portion 1700. Micro controller 1701 receives channelassignment information from system control unit 1360 and utilizes thatinformation to select wavelength, phase and frequency of assignedchannels. Transmitter portion 1700 is coupled to laser reference network1370 and receives multiple wavelength signals from laser referencesource 1360. A polarization controller 1703 under control of microcontroller 1701 selects polarization of the reference laser signals. Theoutput of polarization controller 1703 is coupled to tunable filter1707. Micro controller 1701 via wavelength tuning module 1705 controlstunable filter 1707. Wavelength tuning module 1705 selects a wavelengthin response to micro controller 1701 providing a wavelength selectsignal. Tunable filter 1707 selects the wavelength optical signal fortransmitting data under control of micro controller 1701. A coupler 1709couples the output of tunable filter 1707 to phase selector forselecting one out of “” phases. The phase selector includes a 1×n switch1771 that is controlled by micro controller 1701. Each of the N outputsof switch 1771 is coupled to a corresponding phase modulator 1775.Frequency selection is provided by micro controller 1701 controlling avoltage controlled oscillator 1715. The selected frequency output ofvoltage controlled oscillator 1715 is combined with data to betransmitted D_(TX) by mixer/driver 1717. The data D_(TX) to betransmitted is received from a user node 1331. A filter/switch module1770 under control of micro controller 1701 provides the output ofmixer/driver 1717 t the N phase modulators 1775. Each phase modulator1775 is coupled to a phase delay module 1777. The outputs of the phasedelay modules are the N phases Φ1 through ΦN. Switch 1779 under controlof micro controller 1701 selects the output phase. The output of switch1779 and the wavelength-selected reference are combined in coupler 119and filtered by tunable wavelength filter 1721. Micro controller 1701via wavelength tuning module 1705 controls tunable filter 1721. Theoutput of filter 1721 is the wavelength/frequency/phase selected opticalsignal modulated with transmit data and is coupled to optical network1101. A portion of the output is coupled to a detector 1723 thatprovides a dc feedback signal to micro controller 1701.

FIG. 18 depicts optical network processor receive portion 1800 for theabove described second embodiment. Receive portion 1800 includes aprocessor or micro controller 1801 that provides program controlledoperation of optical network processor receive portion. In addition,micro controller 1801 receives channel assignment information fromsystem control unit 1360 and utilizes that information to select channelwavelength, phase and frequency to select a desired channel for recoveryof received data. The received data is provided to a node 1331. Microcontroller 1801 generates wavelength select, phase select and frequencyselect signals. The frequency select signals control avoltage-controlled oscillator 1815 to provide a frequency selectedsignal to a mixer/driver circuit 1817. The output of mixer/driver 1817is filtered by filter 1840 to provide output data signals D_(TX).Receive portion 1800 is coupled to network 1101 and receives opticalsignals carrying data D_(TX) from another node coupled to network 1101.A depolarizer 1803 depolarizes the optical signals received via network1101. As those skilled in the art will appreciate, depolarizer 1803 maybe replaced with a polarization controller controlled by microcontroller 1801. The output of depolarizer 1803 is coupled to tunablefilter 1807. Micro controller 1801 via wavelength tuning module 1805operates tunable filter 1807. Wavelength tuning module 1805 selects awavelength in response to micro controller 1801 and tunes filter 1807 tothe selected wavelength. Phase selection is accomplished by microcontroller 1801 providing phase select signals to control switches 1871and 1879. Switches 1871 and 1879 are used to select one phase delay pathfrom a group of “n” phase delay, where “n” is the number of selectablephases. Each phase delay path includes a phase modulator 1875 and aphase delay circuit 1877. Micro controller 1871 via bias control 1811controls phase modulators 1875. The output of the selected phase path iscoupled via switch 1879 to coupler 1819. A phase reference signal iscoupled from signals received from network 1101 from coupler 1871 tocoupler 1919 via optical connection 1873. Coupler 1819 combines thephase reference signal from connection 1873 with the output of phaseswitch 1879. The combined output is applied to wavelength filter 1821that is tuned to the wavelength selected by micro controller 1801. Theoutput of tunable filter 1821 is coupled to detector 1823 that separatesan RF signal and a dc servo feedback signal. The RF signal is applied tomixer/driver 1817 via pre amplifier 1880. All of the components shownwithin box 1881 may be fabricated on a single integrated optic chipusing reflective design.

A comparison of transmit portion 1700 of FIG. 17 and receive portion1800 of FIG. 18 shows that much of the functionality of the transmitportion and receive portion is similar. FIG. 19 is a block diagram of atransceiver 1900 in which economies are achieved by utilizing thecommonality of receive and transmit portions, 1800, 1700. Transceiver1900 receives data D_(TX) from a node 1331 and provides data D_(RX) to anode 1331. Transceiver 1900 is coupled to optical network 1101 andreference network 1370 by circulator 1940. A micro controller 1901provides program controlled operation of transmit and receive functions.In addition, micro controller 1901 provides wavelength, phase andfrequency selection to select a desired channel for recovery of receiveddata and providing the received data to a node 1331 and for receipt oftransmit data from node 1331 for transmission over network 1101. Microcontroller 1901 generates wavelength select, phase select and frequencyselect signals for transmit and receive. The frequency select signalscontrol a voltage-controlled oscillator 1915 a to provide a frequencyselected signal to a mixer/driver circuit 1917 a. The output ofmixer/driver circuit 1917 a is filtered by low pass filter 1940 toprovide output data signals D_(RX).

Frequency select signals from micro controller 1901 are used fortransmission of data from a node 1331 over network 1101. Frequencyselect signals control voltage controlled oscillator 1915 to select adesired transmit channel frequency. A mixer/driver 1917 combines theoutput of voltage-controlled oscillator 1915 and D_(TX). The modulatedfrequency signals are applied to filter switch 1970. Micro controller1901 also controls phase and wavelength selections. Phase selection isprovide by micro controller 1910 providing phase selection signals to aphase control module 1972, bias control signals to bias control circuit1911 and filter control signals to filter switch 1970. For transmitdata, filter switch 1970 is active but bias control 1911 is not.Integrated optical chip assembly 1981 provides wavelength selection andphase multiplex selections. Integrated optical chip assembly 1981utilizes reflective multiplex technology. Double pass operation of theintegrated optical chip assembly 1981 greatly enhances performance ofthe wavelength filter operation. Sidelobe suppression is increased, forexample, from 15 dB to 30 dB. Input signals received from network 1101via circulator 1940 are applied to depolarizer 1903. Outputs ofdepolarizer 1903 are applied to a TE polarizer 1982. Polarizer 1982 iscoupled to tunable wavelength filter 1983. Tunable filter 1983 iscoupled to TM polarizer 1984. TM polarizer 1984 is coupled to a phaseselection circuit including 2×2 coupler 1909, a 1×4 optical switch 1985,bias modulator array 1986, phase modulator array 1987 and phase delayand recovery reflection mirror 1988. In the embodiment shown, selectionof four phase channels may be accomplished. The phase selection circuitmay be expanded to more phase channels, but for purposes of drawingclarity, only a four phase channel selection structure is shown. Forboth transmit and receive, micro controller 1901 provides wavelengthselection signals to wavelength tuning module 1905. Wavelength tuningmodule 1905 controls tunable filter 1983 to select the wavelengthchannel for transmit and receive.

For transmit functionality, micro controller 1901 controls filter switch1970 to control the phase modulator array 1987. For receivefunctionality, micro controller 1901 controls bias control 1911 to inturn control bias modulator array 1986. For transmit functionality,filter switch 1970 is used to select a phase and couple the output ofmixer/driver 1917 via coupler 1909 through polarizer 1983, wavelengthfilter 1983, polarizer 1982 to depolarizer 1903 and to network 1101 viacirculator 1940. For receive functionality, optical signals receivedfrom network 1101 are coupled via circulator 1940 through depolarizer1903 to polarizer 1982, tunable filter 1983, polarizer 1984 to the phaseselector. Bias control module 1911 under control of micro controller1901 sets the bias to a quadrature point to stabilize the receive phasechannel. The output of the phase selector is coupled to detector 1923.Detector 1923 provides an RF output to preamplifier 1980. Preamplifier1980 is coupled to mixer/driver 1917, and its output is filtered bylowpass filter 1940 to provide output data to node 1331. The inventionhas been described in conjunction with specific embodiments.

Reference Laser

Multiple lasers may be assembled together to provide a laser referencesource useable in the optical networks and optical communication systemof the invention. Various laser sources may be employed; however, eachlaser source must have specific characteristics. In particular, multiplewavelength lasers that have high launch power are desirable. Inparticular, it is desirable that the reference provides optical signalsfor each wavelength channel at levels greater than 20 mw and each lasersource should desirably meet this requirement. It is also desirable thatnonlinear effects such as self phase modulation (SPM), stimulatedBrillion scattering (SBS) and four wave mixing be minimized. A shortcoherence length of less than 5 mm should be provided for phasemultiplex/switching operation. To ensure proper wavelength multiplexing,wavelength stability is to be controlled within 20 picometers. It isdesirable that spurious spectral components be minimized betweenwavelength channels. In particularly advantageous embodiments of theinvention, 16 to 32 wavelengths are provided by the reference lasersource.

FIG. 20 depicts one embodiment of a multiple wavelength reference lasersource in which multiple distributed feedback (DFB) lasers are used. Aseparate DFB laser is used to generate each wavelength λ₁ through λ_(M).There are two limitations on DFB lasers that need to be accommodated.First, the output of each DFB laser 2001 typically has a narrowlinewidth of less than 50 MHz. This spectral width is too narrow for usein the embodiments of the invention described above. Second, thecoherence length of the DFB output is too large for application in theembodiments of the present invention. Phase modulating the output of theDFB laser with an RF signal broadens the spectral width of the outputand further can reduce the coherence length. In other words, for optimumperformance, the laser signals can not be too coherent and can not havetoo narrow a line width in the above-described embodiments. A phasemodulator 2003 is coupled to each DFB laser 2001. Modulation is with anRF signal having multiple frequency components that are selected in theRF range of 0.01 to 10.0 GHz. Modulation with a multiple component RFsignal produces a laser signal having a broad line width output ofgreater than 20 GHz. In addition, the phase modulation reduces thecoherence length. Each modulated laser output is filtered to removesidelobes by utilizing fiber gratings 2005 to shape the modulated laseroutput spectrum. A DWDM multiplexer 2007 is utilized to combine theoutputs of each of the DFB lasers. Amplifier 2009 amplifies theresulting multiple wavelength laser output. Amplifier 2009 is an EDFA.

FIG. 21 illustrates a modification to the multiple wavelength lasersource of FIG. 20. In the design of FIG. 21, the phase modulated lasersignals at the different wavelengths are each amplified by EDFAamplifiers 2101 prior to being shaped by fiber gratings 2005. Byamplifying each wavelength component prior to combining the wavelengthcomponents, it I possible to achieve a combined output in which thecomponents are more uniform and a higher output level may b achieved.

FIG. 22 illustrates a third embodiment of a multiple wavelength lasersource that may be used in accordance with the invention. SeparateErbium Doped Fiber Lasers (EDFL) 2201 are used as sources. Wavelengthcontrol technology is used to control EDFL emission wavelength. EachEDFL provides a single wavelength output. Fiber gratings 2203 are usedto provide output spectrum shaping and coherence function. EDFAs 2205amplify each output and a DWDM multiplexer 2207 is used to combine theoutputs to produce a multiple wavelength laser output. By using EDFLs,phase modulation is not necessary because the EDFLs have a broader linewidth and the coherence length is not too short. Through selection ofappropriate fiber gratings the desired spectral response is achieved.

An alternative EDFL based design for the multiple wavelength laserreference is illustrated in FIG. 23. In the reference source of FIG. 23,rather than separately amplify each wavelength, a single EDFA amplifier2301 is utilized to amplify the combined output. A filter 2303 is usedto shape the amplified multiplexed output.

FIGS. 24 and 25 illustrate EDFLs suitable for application to the laserreference sources depicted in FIGS. 22 and 23. In the EDFLs of bothFIGS. 23 and 24, an erbium doped fiber 2401 is pumped from a laser pumpsource 2407. The fiber 2401 is coupled at either end to a fiber grating.In the embodiment of FIG. 24, both gratings 2403 and 2405 are reflectingnarowband gratings at the same wavelength. In the embodiment of FIG. 25the narrow band fiber grating 2403 is replaced with a broadbandreflecting grating 2501 or alternatively, a mirror. WDM 2409 couples thepump source 2407 output to fiber grating 2405. An isolator 2411 is usedat the output of the EDFL.

FIG. 26 depicts an optical add/drop 1307 that is utilized to particularadvantage in the embodiments of the invention described above. Inaddition, FIG. 26 also shows further details of a typical EDFAconstruction, such as EDFA 1313. The design shown is for a reciprocaloptical add/drop inserted into optical link network 1101. Opticaladd/drop 1307 utilizes three couplers 2603, 2605, 2607 and two isolators2609, 2611 all of which are known in the art and are commerciallyavailable. Optical add/drop 1307 includes a first bidirectional port P1,a second bidirectional port P2 and a third bi-directional port P3.Bi-directional ports P1 and P2 are connected to optical link network1101 and bi-directional port P3 is coupled to an optical networkprocessor or coupler via bi-directional amplifier 1313. Drop signalsfrom optical link network 1101 are coupled from coupler 2605 to coupler2607 and to isolator 2611. Isolator 2611 couples the optical signals toamplifier 1313. Add signals from amplifier 1313 are supplied to isolator2609. From isolator 2609, the transmit signals are supplied to coupler2607 which in turn is connected to coupler 2603 and from coupler 2603 tooptical link network 1101. A through path couples the couplers 2603 and2605. Coupler 2607 is utilized to permit the bi-directional drop and addof optical signals. Each of couplers 2603 and 2605 are chosen in theillustrative embodiment such that 5% of the optical signal is coupled toan add/drop path and 95% of the optical signal is passed on the throughpath of the coupler. Coupler 2607 is chosen such that 50% of the signalis coupled from one path to the other. Isolators 2609, 2611 are used toprovide directionality for the add and drop paths to the ONP or coupler.Amplifier 1331 comprises an EDFA 1313 a for amplifying output signalsand an EDFA 1313 b for amplifying input signals. A circulator 1313 chaving three ports c1, c2, c3 is used to couple both EDFAs 1313 a, 1313b to the optical network processor or coupler. Drop signals from P1 areextracted via coupler 2603 and are coupled via coupler 2607 to isolator2611, amplified by EDFA 1313 b, applied to circulator 1313 c at its portc2 and extracted from circulator at port c3 which is connected to anoptical network processor at port P3.

Optical signals at port P2 are coupled by coupler 2605 to coupler 2607and processed as described above. Optical signals received at port P3are provided by circulator 1313 c to EDFA 1313 and applied to isolator2609. The output in this add path is applied to coupler 2607 provides50% of the add signal to each of couplers 2603, 2605. Because the samelevel of signals are achieved in transmission of signals from ports P1to P2 as are achieved from ports P2 to P1 and between any combination ofpairs of the three ports P1, P2, P3, the optical add/drop in thisembodiment may be characterized as a reciprocal add/drop.

It will be appreciated by those skilled in the art that various changesand modifications may be made to the various embodiments withoutdeparting from the spirit or scope of the invention. It is intended thatthose various changes and modifications be included within the scope ofthe invention. It is further intended that the invention not be limitedto the various embodiments shown and described herein nor limited tothose embodiments that would be apparent as of the filing date of thisapplication. It is intended that the invention be limited in scope onlyby the claims appended hereto.

What is claimed is:
 1. A method of operating an optical network couplinga plurality of nodes, comprising: providing a plurality of opticalcommunications channels, each channel being determined by three opticalsignal variables, a first one of said variables being a wavelengthselected from a plurality of predetermined wavelengths, a second one ofsaid variables being a phase selected from a plurality of predeterminedphases, and a third one of said variables being a modulation frequencyselected from a plurality of modulation frequencies; and utilizing saidoptical communication channels for communication between said pluralityof nodes.
 2. A method in accordance with claim 1, comprising:determining each said channel from selected first ones of said variablesand from selected second ones of said second variables.
 3. A method inaccordance with claim 1, comprising: determining each said channel fromselected first ones of said first variable, and from selected ones ofsaid second variables, and from selected third ones of said thirdvariables.
 4. A method of improving channel capacity and bandwidth ofoptical fiber networks comprising a plurality of nodes each coupleableto a network comprising optical fiber disposed between said plurality ofnodes, said method comprising: providing communications channels fortransmission of information over said optical fiber network; anddefining each of said communication channels by a selected wavelengthselected from a plurality of selected wavelengths and by a selectedmodulation frequency selected from a plurality of selected modulationfrequencies and a selected phase selected from a plurality of selectedoptical phases.
 5. A method in accordance with claim 4, comprising:providing a source of reference signals; and synchronizing each of saidchannels to said source of reference signals.
 6. A method in accordancewith claim 4, comprising: defining each of said communication channelsby a selected wavelength selected from a plurality of selectedwavelengths and by a selected modulation frequency selected from aplurality of selected modulation frequencies and a selected phaseselected from a plurality of selected optical phases.
 7. A method inaccordance with claim 6, comprising: providing a source of referencesignals; and synchronizing each of said channels to said source ofreference signals.
 8. A method in accordance with claim 4, comprising:providing a laser source as a source of optical reference signals; andsynchronizing each of said channels to said source of reference signals.9. A method in accordance with claim 8, comprising: distributing saidreference signals to each of said nodes.
 10. A method in accordance withclaim 4, wherein: said network is in a ring configuration.
 11. A methodin accordance with claim 4, wherein: said network is in a starconfiguration.
 12. A method in accordance with claim 4, wherein: saidnetwork is in a mesh configuration.
 13. A method in accordance withclaim 5, comprising: distributing said reference signals to said nodesvia a distribution.